2025A&A...702L...3S

Aims. We aim to characterize the physical and activity properties of the interstellar comet 3I/ATLAS through spectroscopic and photometric observations during the first month after its discovery.
Methods. We performed time series photometry and long-slit spectroscopy between 2 and 29 July 2025 using multiple ground-based telescopes. Photometric data were calibrated against field stars from the ATLAS and APASS catalogs, and Fourier analysis was applied to derive the comet's rotational period. Spectral data were obtained using the Southern African Large Telescope and the Nordic Optical Telescope.
Results. We report a spin period of 16.16 ± 0.01 h with a light curve amplitude of approximately 0.3 mag. The comet exhibits increasing dust activity and reddening colors during the observation period with no visible tail detected, likely due to viewing geometry and low dust production. Dust mass-loss rates are estimated between 0.3 and 4.2 kg s^–1, consistent with weakly active distant comets. Spectral colors are similar to those of outer Solar System comets and differ from previously reported values for 3I/ATLAS.
Conclusions. The morphological and photometric properties of 3I/ATLAS are consistent with a weakly active comet of outer Solar System origin despite its interstellar provenance. Continued monitoring around perihelion is necessary to track changes in activity and color, which will provide insights into the evolution of interstellar materials under solar radiation.